1. Field of the Invention
The present invention relates to a source driver for an active matrix liquid crystal display panel.
2. Description of the Related Art
It is known that AC driving is required to guarantee long-term reliability of a Liquid Crystal Display (LCD) panel by preventing properties of LCD materials of the LCD panel from being degraded. Accordingly, in a conventional active matrix LCD device, a drive voltage according to a gray level of image data is supplied by a source driver between the electrodes of an LCD element of each cell (pixel) of an LCD panel, and the polarity of this drive voltage is inverted, for example, every frame of an image signal with respect to a reference potential (refer to Japanese Laid-open Patent Application No. 2005-201974). For example, if a drive voltage of an H (high potential) side relative to the reference potential is supplied in a frame, a drive voltage of an L (low potential) side relative to the reference potential is supplied in the next frame.
Instead of simultaneously setting all cells of the LCD panel to a drive voltage of the same side with respect to the reference potential, a dot inversion driving scheme or a 2-line dot inversion driving scheme is used. In the dot inversion driving scheme, neighboring cells in rows and columns have drive voltages of opposite sides with respect to the reference potential. In the 2-line dot inversion scheme, neighboring cells in columns have drive voltages of opposite sides and neighboring cells in rows have drive voltages inverted every two lines.
FIG. 1 illustrates as a example a waveform of a signal supplied to one of a plurality of column terminals of a LCD panel from a source driver. A reference potential (VCOM) signal is a potential of one terminal of a two-terminal LCD element and is a constant DC voltage (for example, 6V). The VCOM signal typically has a potential corresponding to approximately ½ an output voltage of a driving power source. A drive voltage is inverted in polarity with respect to the VCOM signal at a line scan period (scan period per line) like a characteristic denoted by solid line AL in FIG. 1. A waveform denoted by dashed lines BL in FIG. 1 shows a drive voltage supplied to an LCD element through an adjacent column terminal in the dot inversion driving scheme. Although the drive voltage is a voltage according to a gray level of image data as described above, the drive voltage after inversion becomes constant (that is, becomes the same gray level value) in FIG. 1.
Recently, in an LCD device, high-speed operation is required for improved picture quality. For this requirement, it is necessary to charge an LCD element of an LCD panel at high write speed by a drive voltage according to a gray level indicted by image data. On the other hand, power consumption of the device or heating amount of the driver, which have trade-off relations with the high write speed, need to be decreased.